1. Field of the Invention
The present invention pertains to liquid silicone rubber base compositions comprising an organopolysiloxane with multiple aliphatically unsaturated functional groups and a pyrogenic silica filler, and in particular to a process for preparing liquid silicone rubber base compositions with improved whiteness, in a reproducible manner.
2. Description of the Related Art
Liquid silicone rubber (“LSR”) is now widely used commercially, for example for the molding of films and articles, coatings, caulks, sealants, and the like. LSR contains at least one organopolysiloxane bearing multiple aliphatically unsaturated groups such as vinyl groups, allyl groups, higher 1-alkenyl groups (meth)acryloyl groups, ethynyl groups, and the like. To achieve high strength after crosslinking to a solid, relatively large amounts of reinforcing fillers are added. The compositions may also contain conventional components such as low molecular weight crosslinkers, adhesion promoters, dyes, pigments, etc.
The base polymer composition may be crosslinked by addition of a free radical initiating catalyst; photochemically crosslinked by addition of a suitable photocuring catalyst (curing then taking place upon exposure to actinic radiation, e.g. UV light); or may be mixed with an Si—H functional crosslinker and hydrosilylation catalyst. These systems may be prepared as one component or two component systems. An inhibitor is often added to systems which cure by hydrosilylation to promote storage stability or to increase pot life. The LSR base polymer composition is common to all these systems, and great demand is placed on its quality, including color, viscosity, viscosity change during storage, homogeneity, and the like. In addition, the base polymer composition must be tailored to provide suitable physical characteristics of the cured polymer, including hardness, resiliency, compression set, tensile strength, modulus, elongation, tear strength, tear propagation strength, etc.
A principle problem which affects numerous characteristics of both the base rubber polymer and the cured product, is incorporation of the reinforcing filler, which is often present in large amounts. Pyrogenic silica (fumed silica) is often used as a reinforcing filler, which requires a specific surface area (BET) of greater than 50 m2/g. While colloidal silica also meets this surface area criterion, colloidal silica, due to its wet preparation method, is much more hydrophilic than fumed silica, and hence the latter is the reinforcing filler of choice.
However, fumed silica is also relatively hydrophilic, due to its content of surface silanol groups. Homogeneously dispersing such fillers into a very hydrophobic silicone is exceptionally difficult, and requires numerous mixing stages, each operating for significant periods of time. Even so, the resultant mixture is still often inhomogeneous, sometimes even with macroscopic domains of poorly incorporated filler.
To avoid such problems, hydrophobing the filler has long been practiced. Two methods of hydrophobing are known. In the first method, “ex situ” hydrophobing, fumed silica is reacted with a hydrophobing agent such as an alkoxyalkylsilane, chloroalkylsilane, alkyldisilazane, akoxy- or hydroxyl-functional oligo- or polysiloxane or a cyclopolysiloxane. The silica thus “loaded” or even partially reacted, is then “reacted” at elevated temperature, following which the hydrophobic silica is stripped of unreacted hydrophobing agents, and dried. Ex situ silica hydrophobizing is disclosed, for example, in U.S. Pat. Nos. 2,938,009; 3,334,062; 3,397,220; and 3,635,743.
In the U.S. Pat. No. 3,635,743 patent, the fumed silica is first treated with ammonia or an organic amine, followed by reaction with the hydrophobing reagent. A preferred hydrophobing reagent is hexamethyldisilazane. The silica must contain adsorbed or absorbed water, and sufficient additional water is added to ensure complete hydrolysis and reaction of the hexamethyldisilazane. The silica is agitated several hours at 130° C. and dried in an oven for 24 hours at 150° C. The dry pulverulent product contains substantially no silanol groups, these having been etherified with trimethylsilyl groups.
Such pulverulent hydrophobic silicas are much easier to disperse in hydrophobic media of all types, including aliphatically unsaturated silicones. However, the preparation method involves a long processing time at elevated temperature (including drying and stripping of unreacted hydrophobing agents), and transportation of the product is not particularly economical due to its low density, unless the product is compacted prior to shipment. This, however, then requires the product to be “decompacted” at the point of use, a separate step, and includes the risk that some portions may remain in compacted or partially compacted form, thus forming inhomogeneous domains in the product.
Thus, “in situ” methods of hydrophobing silica have been developed. In the in situ method, hydrophilic fumed silica is added to the hydrophobic continuous phase, e.g. an aliphatically unsaturated silicone, and a hydrophobing agent is added. The slurry of silica and hydrophobing reagent in the hydrophobic continuous phase is intensively mixed, and heated to a temperature sufficient for hydrophobing to be effected, for example 150-160° C., and then cooled. At this point, additional composition ingredients such as additional aliphatically unsaturated silicone are added, and the mixture is homogenized in at least one, but often in a plurality of mixers or kneaders.
A problem which exists with such in situ methods is “structuring”, which is believed to be caused by interaction of the filler and the continuous phase, resulting in an undesired high viscosity, or even thixotropy. Furthermore, it has been found that in situ methods frequently produce base polymer compositions where the color is off, i.e. a significant and visually observable coloration, generally yellow (Y* component of the color index) is present. Furthermore, areas of microinhomogeneity are often present, which affects transparency as well as product properties such as tear strength, elongation, and tear propagation strength (crack arrestment).
In U.S. Pat. No. 5,506,303, for example, the problems of high viscosity and crack propagation are particularly noted with respect to in situ hydrophobing. The U.S. Pat. No. 5,506,303 patent proposes to overcome these deficiencies by adding, in a first step, an untreated (hydrophilic) fumed silica to a first portion of silicone base polymer while maintaining the temperature below 80° C., optionally with a hydrophobing agent such as a silazane, then, in a second step, mixing a further portion of silicone base polymer with the product of the first step, and only then thermally treating the mixture obtained in the second step, to provide a silicone base polymer composition of low viscosity and good crack propagation resistance.
However, U.S. Pat. No. 5,506,303 cautions that the base polymer must be added in two distinct portions, and also cautions that it is exceedingly important that the initial (first step) and second step mixtures must be held to less than 80° C. during the mixing procedure and at all times prior to thermal treatment. For example, a composition where all the silicone oil was added in one step had a viscosity of ca. 200,000 poise, as compared to ca. 7000 poise when two step addition was used. If the temperature during the first mixing step was raised to the reaction temperature of the hydrophobing reagent prior to addition of additional base polymer, the hardness, elongation, tear strength, and crack arrestment properties all declined substantially. However, even when the procedure disclosed by U.S. Pat. No. 5,506,303 is used, the product color is often observably discolored, which is undesirable.
U.S. Pat. No. 6,288,143 is not directed to the same field of endeavor as applicants, but acknowledges that in filler-containing condensation curable compositions, long mixing times of 3 to 5 days are required. The U.S. Pat. No. 6,288,143 patent addresses this problem by an in situ master batch process, where a highly filler-loaded master batch of silica in a non-reactive silicone oil, i.e. a trimethylsilyl-capped polydimethylsiloxane, is prepared, and this master batch is then diluted with a reactive, condensation curable (moisture curable) silicone to produce an RTV-1 composition. However, the use of a non-reactive silicone in the master batch decreases the obtainable tensile strength and modulus, and in some cases, the non-reactive silicone can exude or “sweat” from the cured product. This method is thus generally unacceptable for addition-curable systems.
It would be desirable to provide a process for the preparation of filler-containing silicone base polymer compositions useful as a component in addition-curable LSR systems which avoids the problems of the prior art. It would be most desirable to provide for a one pot preparation where lengthy cycle times are avoided, where the base polymer composition is of low viscosity, the silicone rubber base composition is substantially free of discoloration, e.g. is white in color, and which provides cured products with excellent physical properties.